Losing Users on Logon page

I've recently bought a new machine with Windows XP on it. I installed ZoneAlarm and disabled all default passwords and all the other security checks I could think of. I then set up accounts for each of the family members and had them set passwords on the accounts. After using the machine for a few days I've noticed that almost every time one of my users logs on all the other users on the machine get locked out. Now I intentionally turned on lock out after three tries on the system and that, I think , is what is locking the others users out. I've checked everything I can think of associated with that user to see what may be doing this in his account but have come up with nothing so far. It happens both connected to the internet and not. Does anyone have an idea what might be causing this or where to look for malicious code?